• part one 7.99.6 drm_debug=1 with diff
• part two dmesg with filename with diff-with-filename

List of Files to check

sys/external/bsd/drm2/i915drm/i915_pci.c
sys/external/bsd/drm2/dist/drm/i915/intel_display.c
sys/external/bsd/drm2/i915drm/intelfb.c

genfb_attach -> setmode callback -> drm_fb_helper_set_config

sys/external/bsd/drm2/dist/drm/drm_fb_helper.c

Following still gives strange screen

diff --git a/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c b/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
index 1a727d1..54eb0c9 100644
--- a/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
+++ b/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
@@ -864,7 +864,7 @@ drm_fb_helper_set_config(struct drm_fb_helper *fb_helper)
 	drm_modeset_lock_all(dev);
 	drm_fb_helper_restore_fbdev_mode(fb_helper);
 	drm_modeset_unlock_all(dev);
-
+	while (1) { };
 	if (fb_helper->delayed_hotplug) {
 		fb_helper->delayed_hotplug = false;
 		drm_fb_helper_hotplug_event(fb_helper);

but Following gives blank screen

diff --git a/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c b/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
index 1a727d1..6701057 100644
--- a/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
+++ b/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
@@ -860,7 +860,7 @@ int
 drm_fb_helper_set_config(struct drm_fb_helper *fb_helper)
 {
 	struct drm_device *dev = fb_helper->dev;
-
+	while (1) { };
 	drm_modeset_lock_all(dev);
	drm_fb_helper_restore_fbdev_mode(fb_helper);
 	drm_modeset_unlock_all(dev);

The suspect is at:

drm_fb_helper_restore_fbdev_mode(fb_helper);

Following gives blank screen
diff --git a/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c b/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c

index 1a727d1..5f500bc 100644
--- a/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
+++ b/sys/external/bsd/drm2/dist/drm/drm_fb_helper.c
@@ -321,7 +321,7 @@ bool drm_fb_helper_restore_fbdev_mode(struct drm_fb_helper *fb_helper)
 	list_for_each_entry(plane, &dev->mode_config.plane_list, head)
 		if (plane->type != DRM_PLANE_TYPE_PRIMARY)
 			drm_plane_force_disable(plane);
-
+	while (1){};
 	for (i = 0; i < fb_helper->crtc_count; i++) {
 		struct drm_mode_set *mode_set = &fb_helper->crtc_info[i].mode_set;
 		struct drm_crtc *crtc = mode_set->crtc;